Agricultural films which are largely used in greenhouse culture or tunnel culture chiefly include soft ethylene resin films which are about 30 to 200 .mu.m thick and which comprise, as a base resin, polyvinyl chloride (hereinafter abbreviated as PVC), branched low-density polyethylene (hereinafter abbreviated as LDPE), ethylene-vinyl acetate copolymers (hereinafter abbreviated as EVA), linear low-density polyethylene (hereinafter abbreviated as LLDPE), etc. Of the various properties required for the agricultural films, particularly important are weather resistance, anti-fogging properties, heat-retaining properties, and transparency. To cope with the recent situation confronting agriculture such as an increased cost and a shortage of labor, development of films having an extended duration of life before re-placement is desired.
Deterioration in elongation of agricultural films occurs through exposure to ultraviolet rays of sunlight in the presence of oxygen in the air and they break. In order to obtain good weathering, various proposals have so far been made to provide agricultural films having an extended use-life and improved performance properties against weathering. For example, various weathering stabilizers to be incorporated into plastic compounds have hitherto been proposed.
Weathering stabilizers can be roughly divided into ultraviolet absorbents and light stabilizers. In recent years, light stabilizers have taken the lead because they are more effective in thin agricultural films in improving weather resistance with a smaller amount, which leads to a reduction in cost.
While nickel-containing light stabilizers have conventionally been used, many hindered amine compounds have recently been proposed as light stabilizers and are now widely employed because they produce excellent effects at reduced amounts.
However, hindered amine light stabilizers generally have a small molecular weight and are readily released from the film, thus failing to retain their stabilizing effects for an extended period of time.
In addition, since hindered amine light stabilizers having excellent light stabilizing effects are alkaline, if they are brought into contact with insecticides used for extermination of harmful insects, such as chlorinated organic compounds and organophosphorus compounds, chemical reactions with acidic substances resulting from decomposition of these insecticidal compounds occur thereby weakening their effects as weathering stabilizers.
In order to overcome the above-described problem, use of hydrotalcite as a neutralizing agent in combination with hindered amine light stabilizers, is disclosed in JP-A-63-175072 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the approach is not a completely satisfactory solution to the above-mentioned problem that the hindered amine light stabilizers are unavoidably released from the film because of their low molecular weight.
The atmosphere within greenhouses or tunnels surrounded by an agricultural film is saturated with water vapor which evaporates from the soil or plants, and the water vapor dropwise condenses on the inner surface of a cold film to cause fogging. Water droplets on the film not only greatly reduce the incident sunlight due to irregular reflection but the droplets fall on the plants resulting in frequent occurrence of diseases.
To avoid fogging, anti-fog treatments on the inner surface of an agricultural film converting the film into an anti-stick water film and preventing adhesion of water droplets have been used thereby assuring transmission of sunlight into the greenhouse or tunnel, raising the soil temperature and air temperature within the greenhouse or tunnel, accelerating photosynthesis of the plants, accelerating healthy growth of the plants, and controlling the occurrence of plant diseases.
Currently employed anti-fog treatments include incorporation of an anti-fogging agent (anti-fog additive) into a film-forming ethylene resin compound and coating of an anti-fogging agent on a film.
However, an anti-fogging film obtained by coating a soft plastic film with an anti-fogging agent has not yet been employed practically as an agricultural film for the following reasons. Because of their low surface energy, soft plastic films for agricultural use generally have poor wettability and adhesion when coated with surface active agents or hydrophilic high polymeric substances which have been used as anti-fogging agents. This tendency is particularly conspicuous with soft ethylene resin films of low polarity, e.g., LDPE, EVA, and LLDPE films. Therefore, where an anti-fogging agent is spray coated with a power atomizer onto a soft ethylene resin film, the anti-fogging agent needs to be used in a large quantity and this increases cost, and a large amount of time is required for spray coating operation. Further, spray coating cannot be effected uniformly with insufficient anti-fogging effects arising. Where an anti-fogging agent is applied using a coater, etc., a large quantity of a coating is consumed, and the coating speed cannot be increased, resulting in an increase of cost. In either case, the coated anti-fogging agent is washed away together with running water droplets due to poor adhesion resulting in a very short life for the anti-fogging properties. Furthermore, the coated film undergoes blocking due to the stickiness of the anti-fogging agent. As a result, it has been impossible to retain anti-fogging effects in a stable manner for a long duration of at least 1 year, more desirably, several years. Most of the state-of-the-art agricultural films exhibit anti-fogging properties for a period of only about 1 month.
Anti-fogging agents commonly incorporated into the films include nonionic, anionic and cationic surface active agents.
Other methods for providing anti-fogging properties to agricultural films, in addition to the coating method and incorporation method, include chemical modification of the ethylene base resin or the ethylene resin film surface by introducing a polar group, such as a hydrophilic group. This technique, however, entails high cost at the present time and is difficult to apply to agricultural films.
On the other hand, coating type anti-fogging agents mainly comprising an inorganic hydrophilic colloidal substance and a hydrophilic organic compound have recently been developed as disclosed, e.g., in JP-B-63-45432, JP-B-63-45717, and JP-B-64-2158 (the term "JP-B" as used herein means an "examined published Japanese patent application"). Suitable inorganic hydrophilic colloidal substances include colloidal silica, colloidal alumina, colloidal Fe(OH).sub.2, colloidal Sn(OH).sub.4, colloidal TiO.sub.2, colloidal BaSO.sub.4, and colloidal lithium silicate, with colloidal silica and colloidal alumina most generally used. Suitable hydrophilic organic compounds include various nonionic, anionic or cationic surface active agents; graft copolymers mainly comprising a hydroxyl-containing vinyl monomer unit and from 0.1 to 40% by weight of a carboxyl-containing vinyl monomer unit or a partial or complete neutralization product thereof; and sulfo-containing polyester resins.
Coating type anti-fogging agents used with this new film type have markedly improved wettability and adhesion and coated agricultural films exhibiting anti-fogging properties for a duration of about a half year to about 1 year are provided.
The temperature in the greenhouse falls when the outside temperature falls at night, particularly at dawn. Taking an integrated temperature (daily mean air temperature .times.days; e.g., 2500.degree. to 3000.degree. C. in the case of rice plant) into consideration, a temperature difference of 1.degree. or 2.degree. C. at dawn greatly influences crop yield and harvest-time. Therefore, heat-retaining property is a particularly important characteristic of agricultural films.
Additives for imparting heat-retaining properties to agricultural films include fillers, such as silica powder (see JP-B-47-13853), magnesium compounds (see JP-B-3-50791), and hydrotalcite (see JP-B-62-31744), water-absorbing resins (see JP-A-61-81446), and ethylene-vinyl alcohol copolymers (see JP-A-55-118941). In particular, hydrotalcite is known to provide a satisfactory balance between heat-retaining properties and transparency in view of its heat retention effect and less impairment of transparency as compared with other inorganic fillers.
Transparency of agricultural films is essential for plants growing in the sun, and agricultural films preferably have as high transparency as possible.
Under these circumstances, none of the state-of-the-art polyolefin-based agricultural films satisfies all of the requirements of heat-retaining properties, transparency, weather resistance, and anti-fogging properties.